Magneto-Structural Properties and Theoretical Studies of a Family of Simple Heterodinuclear Phenoxide/Alkoxide Bridged MnLn Complexes: On the Nature of the Magnetic Exchange and Magnetic Anisotropy.

A family of MnLn strictly dinuclear complexes of general formula [Mn(μ-L)(μ-OMe)(NO)Ln(NO)(MeOH)] (Ln = Gd, Dy, Er, Ho) has been assembled in a one pot synthesis from a polydentate, multipocket aminobis(phenol)ligand [6,6'-{(2-(1-morpholyl)ethylazanediyl)bis(methylene)}bis(2-methoxy-4-methylphenol)], Mn(NO)·4HO, Ln(NO)· nHO, and NEt in MeOH. These compounds represent the first examples of fully structurally and magnetically characterized dinuclear MnLn complexes. Single X-ray diffraction studies reveal that all complexes are isostructural, consisting of neutral dinuclear molecules where the Mn and Ln metal ions, which exhibit distorted octahedral MnNO and distorted LnO coordination spheres, are linked by phenoxide/methoxide double bridging groups. Static magnetic studies show that the MnGd derivative exhibits a weak antiferromagnetic interaction between the metal ions, with a negative axial zero-field splitting D parameter. The MnGd complex shows a notable magnetocaloric effect with magnetic entropy change at 5 T and 3 K of -Δ S = 16.8 J kg K. Theoretical studies were performed to support the sign and magnitude of the magnetic anisotropy of the Mn ion ( ab initio), to predict the value and nature of J, to disclose the mechanism of magnetic coupling, and to establish magneto-structural correlations (DFT calculations). The results of these calculations are corroborated by quantum theory of atoms in molecule analysis (QTAIM). Finally, Mn-Dy and Mn-Er complexes show field-induced slow relaxation of the magnetization but without reaching a maximum above 2 K in the out-of-phase ac susceptibility. Ab initio calculations were also performed on Mn-Dy/Ho systems to unravel the origin behind the weak SMM characteristics of the molecules possessing two strongly anisotropic ions. The mechanism of magnetic relaxation was developed, revealing a large QTM/tunnel splitting at the single-ion level. Furthermore, the anisotropy axes of the Mn and Ln ions were calculated to be noncollinear, leading to reduction of the overall anisotropy in the molecules. Hence, the herein reported complexes demonstrate that a combination of two anisotropic metal ions does not guarantee SMM behavior.